This application is based on Japanese Patent Application No. 76846/1998 filed on Mar. 25, 1998 and No. 83167/1998 filed on Mar. 30, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a method of and apparatus for forming an image on a multilevel gradation, and will be found advantageous when used in forming an image by electrophotography in which exposure is subjected to multi-valued control.
2. Description of the Prior Art
Various types of image-forming apparatuses such as digital copying machines and page printers have been employed for printing out image data. In these apparatuses, it is most common to reproduce half tones on a multilevel gradation by a combination of multi-valued density control for each pixel and quasi-gradation (quasi-continuous tone) control for each pixel matrix. The density control for each pixel is effected, e.g., by pulse width modulation (PWM) and/or intensity modulation to which a light source for use in exposure in electrophotography is subjected, while the technique commonly in use for the quasi-gradation control is an error diffusion method, which is briefly summarized in the following paragraph:
Ordinarily, 4 to 8 gradations can be reproduced for each pixel. 4 and 8 gradations correspond to 2 and 3 bits respectively. This means that only a small number of gradations can be reproduced for each pixel as compared with 256 gradations which correspond to 8 bits and by which an original image data is constituted. Consequently, image processing which has previously been effected in the apparatuses of the kind indicated above is characterized in that an input image data representing an original image is converted into an output image data having a smaller number of bits than the input image data, that this data conversion would give rise to an error in the number of bits used to represent each pixel, and that this error is distributed among pixels disposed around the pixel involved.
Conventionally, one has not failed to see a fixed correspondence between the gradation levels of the input image data and those of the output image data, with the exception of some cases where the marginal zone of the output image data has a different number of gradations from the remaining zone thereof so that the edge sharpness of the original image may be maintained and gradation reproduction in the remaining zone may be allowed to proceed smoothly.
With respect to an electrophotographic image-forming apparatus having a function to form a toner powder image of a prescribed pattern, it is known to subject the toner powder image to automatic image density control (AIDC), in which the conditions for image formation such as electric potential applied to a photoreceptor are adjusted in accordance with the measured value of the image density of the toner powder image.
Because of the aforesaid fixed correspondence between the gradation levels of the input image data and those of the output image data, gradation reproduction obtainable from the prior art image-forming apparatuses depends on the conditions for image formation such as temperature, humidity, ratio of toner to developer, and surface characteristic of photoreceptor. This dependence makes the quality of a reproduced image unstable such that the image density of an image actually printed out differs from that specified by an input image data, the difference therebetween being caused by error diffusion effected on the basis of the gradation level of the output image data irrespective of a change in the image density of a printout, such a change being in turn caused by a change in a photographic situation or conditions for image formation. Especially, in case where the difference between the gradation levels of the input image data and those of the output image data is added to the subsequent input image data, the gradation level of a new output image data is determined by the result of this addition and may yield an irregular, instead of a proper, image density throughout an entire printout. This is the very reason why the image density of an image actually printed out differs from that specified by an input image data. In case of an apparatus for forming a color image from a plurality of toner powder images of various colors, a change in the gradation reproduction is prominently manifested by a change in the reproduced colors.
The present invention is concerned with eliminating the aforesaid difficulty. As such, the primary object of the invention is to provide a method of and apparatus for forming an image capable of effecting constant gradation reproduction and printing out images stable in quality.
According to an embodiment of the invention, correspondence between the gradation levels of the input image data and those of the output image data is varied on the basis of reproduced image densities measured at one or preferably more points in a printout method. In order to vary the aforesaid correspondence on the basis of densities measured at a plurality of points, the image may be partitioned into regions corresponding to these points so that the aforesaid correspondence may be varied in each of these regions. Alternatively, image densities reproduced in positions disposed in areas between these points may be inferred from measured values obtained at these points so that the aforesaid correspondence may be varied in regions into which the image is more finely partitioned.
According to a further feature, a larger number of gradations than those by which an output image data is to be constituted are subjected to arithmetic operations so that the number of gradations corresponding to the difference between a measured value and a desired density may be made sufficiently large and thereby as many choices as possible may be provided in determining the aforesaid correspondence.
In one described embodiment, an apparatus for forming an image is adapted to apply the error diffusion method to an input image data having 2n gradations so as to convert it into an output image data having 2m gradations, where n is an integer which is equal to or more than 3, and m is an integer which is less than n and is equal to or more than 2. Correspondence between N-stage gradation levels of the input image data and M-stage gradation levels of the output image data can be varied to suit requirements, where N and M are equal to 2n and 2m respectively. This apparatus for forming an image has control means for setting the aforesaid correspondence in accordance with data obtained from the measurement of the image densities of a printout having specific gradation levels which do not give rise to errors in the aforesaid conversion.